1. Field of the Invention
The present invention relates generally to an active vibration damping device interposed between two members of a vibration system for actively attenuating or reducing vibrations transmitted between the two members, more particularly to a pneumatically operated active vibration damping device capable of changing its vibration damping or isolating characteristics depending upon vibrations to be damped, by utilizing an air pressure change.
2. Description of the Related Art
A known example of the above type of pneumatically operated active vibration damping device is disclosed in JP-A-10-184769. The disclosed active vibration-damping device includes: a first and a second mounting member connectable to the two members of the vibration system, respectively; an elastic body elastically connecting the first and second mounting members; a pressure receiving chamber partially defined by the elastic body to which a vibrational load is applied; an easily deformable flexible diaphragm partially defining an equilibrium chamber whose volume is variable; a first orifice passage permitting fluid communication between pressure receiving and equilibrium chamber; and an elastic oscillating plate member which partially defines the pressure receiving chamber on one of opposite sides thereof and an oscillating air chamber on the other side thereof. The disclosed pneumatically operated vibration damping device is able to exhibit a passive damping effect based on resonance of the fluid which is forced to flow through the first orifice passage between the pressure receiving chamber and the equilibrium chamber upon application of the vibrational load to the pressure received chamber, and an active damping effect based on oscillation of the elastic oscillating plate actively generated upon application of a periodic air pressure change to the oscillating air chamber.
Described in detail, this type of active vibration damping device may be arranged to exhibit a high passive damping effect with respect to low frequency vibrations based on the resonance of the fluid flowing through the first orifice passage, while being arranged to actively offset or isolate high frequency vibrations based on the oscillation of the elastic oscillating plate, for example. For the above-described advantages, this type of vibration damping device has been utilized as vibration-damping devices, such as an engine mount and a body mount of vehicles, which are required to exhibit desired vibration damping effect with respect to a plurality of frequency ranges or over a wide frequency range.
In the active vibration damping device constructed as described above, a sufficient amount of flow of the fluid through the first orifice passage is required to ensure high damping effect of the device with respect to the low frequency vibrations. To meet this requirement, it may be attempted to give the oscillating elastic plate a relatively large spring stiffness, so as to restrict passive elastic deformation of the oscillating elastic plate, thus minimizing fluid pressure absorption in the pressure-receiving chamber due to the passive elastic deformation of the oscillating elastic plate. This arrangement may ensure a relatively large amount of pressure change of the fluid in pressure-receiving chamber upon application of the vibrational load to the pressure-receiving chamber.
However, the large spring stiffness of the oscillating elastic plate may deteriorate oscillation efficiency of the oscillating elastic plate caused by the periodic air pressure change induced in the oscillating air chamber. This makes it difficult to control the fluid pressure in the pressure-receiving chamber with efficiency, leading to deterioration of active vibration isolating effect of the damping device with respect to the high frequency vibrations.
To cope with this problem, the inventors of the present invention has been proposed a modified active vibration damping device disclosed in JP-A-10-184770. The disclosed damping device further includes: a rigid partition member dividing the pressure receiving chamber into two sections, namely a primary fluid chamber partially defined by the elastic body and an auxiliary fluid chamber partially defined by the elastic oscillating plate; and a second orifice passage permitting a fluid communication between the primary and auxiliary fluid chambers and being tuned so that resonance of the fluid flowing through the second orifice passage exhibits a desired vibration isolating effect with respect to high frequency vibrations. In the proposed vibration-damping device, the periodic air pressure change applied to the oscillating air chamber can be efficiently transmitted by utilizing the resonance of the fluid flowing through the second orifice passage, even in the case where the oscillating elastic plate has relatively large spring stiffness. Thus, the proposed active vibration-damping device is able to improve both of a vibration damping effect with respect to the low frequency vibrations based on the resonance of the fluid flowing through the first orifice passage, and a vibration isolating effect based on a fluid pressure control between the primary and auxiliary fluid chambers.
The extensive studies on the proposed active vibration damping device, which were made by the present inventors have revealed that the efficient transmission of the fluid pressure change between the primary and auxiliary fluid chamber owing to the resonance of the fluid flowing through the second orifice passage is just effective with respect to vibrations in a very limited frequency band, and a resistance to flow of the fluid through the second orifice passage tents to be increased when frequencies of the vibrations become higher than the limited frequency band to which the second orifice passage is tuned, resulting in significant deterioration of the pressure transmitting efficiency between the primary and auxiliary fluid chambers upon application of the higher frequency vibrations. Accordingly, the proposed active vibration-damping device still suffers from an inherent problem of deterioration of its vibration damping or isolating effect with respect to the high frequency vibrations.
Due to the above-described increase of the resistance to flow of the fluid through the second orifice passage, the fluid pressure change in the pressure receiving chamber upon application of the high frequency vibrations is never absorbed by the volumetric change of the auxiliary fluid chamber, as well as the equilibrium chamber, leading to a higher dynamic spring constant of the vibration damping device, resulting in deterioration of the passive vibration damping effect of the vibration damping device.
That is, the proposed pneumatically oscillated active vibration damping device disclosed in JP-A-10-184770 is still insufficient to exhibit a sufficiently vibration damping effect with respect to vibrations over a wide frequency range, namely a low frequency range to which the first orifice passage is tuned, a high frequency range to which the second orifice passage is tuned, and a higher frequency range which is higher than the frequency range to which the second orifice passage is tuned.